An increase free ammonia content was confirmed in Japanese barnyard millet (Echinochloa utilis OHWI), crabgrass (Digitaria adscendens HENR.), livid amaranth (Amaranthus lividus L.) and purple nutsedge (Cyperus rotundus L.) treated with bialaphos, L-2-amino-4-[(hydroxy)(methyl)phosphinoyl]butyryl-L-alanyl-L-alanine. The ammonia content reached about 30 to 100 times higher than the control's in 24 to 48 hr after the treatment. The increment was more marked in shoots than in roots of the barnyard millet, and more in leaves than in stems of livid amaranth. The accumulation was not momentary, but persistent until the death of the plant. Such a remarkable accumulation of ammonia was not observed in plants treated with any other herbicide. Therefore, the ammonia accumulation is considered to be a particular action caused by bialaphos. The close correlation between the free ammonia content and the herbicidal activity indicated that the toxicity of accumulated ammonia is a primary factor to the herbicidal activity of bialaphos.

The activities of enzymes including peroxidase, polyphenoloxidase, phenylalanine ammonia lyase, tyrosine ammonia-lyase or catechol-O-methyltransferase in rice leaves were surveyed to elucidate the mechanism of action of probenazole in connection with the resistant reac tion of host plant against rice blast fungus. The activities of enzymes were increased evidently in the treated-inoculated rice leaves with probenazole and rice blast fungus conidia than in the leaves of treated-noninoculated, nontreated-inoculated or nontreated-noninoculated one. The above results seem to be closely correlated with the facilitated formation of a physical barrier in and around the invaded cell, which was observed microscopically.

After treating rice plants by submerged application with probenazole no changes were noted in the siliciferous cells in the leaves, no morphological changes in the leaf tissue and no anti-fungal activity in leaf exudates were detected. When rice plants were treated by submerged application of probenazole and subsequently inoculated with the blast fungus (Pyricularia oryzae CAVARA) peroxidase activity (oxidation of phenolic substances) increased and an anti-conidial factor in the rice leaf increased. Also the increase of the factor was observed in rice plants treated by the chemical but not inoculated. A part of the appearance of induced resistant characteristics were thought to be similar to a reaction of resistant rice plant. The induction of resistant characteristics were largely inhibited by a heat shock upon rice plant indicating that the event was host mediated reaction. Probenazole (submerged application) appears to control the blast disease in rice by a combination of the events, such as inhibiting sensitive stages in the life cycle of the blast fungus and also activating the host defence.

Bialaphos, L-2-amino-4-[(hydroxy)(methyl)phosphinoyl]butyryl-L-alanyl-L-alanine, at a concentration of 0.3 to 3 ms did not inhibit glutamine synthetase (L-glutamine: ammonia ligase (ADP), EC 6. 3. 1. 2, GS) extracted from shoots of Japanese barnyard millet (Echinochloa utilis OHWI). However, low GS activity was observed in the shoots treated with bialaphos. On the other hand, L-2-amino-4-[(hydroxy) (methyl)phosphinoyl]butyric acid (L-AMPB), a metabolite of bialaphos, inhibited GS both after extraction and in the shoot. The results suggest that bialaphos was metabolized into L-AMPB in plants and inhibited GS. The quantitative analysis of free amino acids in the bialaphos treated shoots showed a remarkable decrease in glutamine content 48 hr after the treatment. An exogenous application of glutamine increased the glutamine to some extent in the bialaphos-treated shoots, but did not antagonize bialaphos in the herbicidal activity. Thus, it appears that a decrease of glutamine content by the herbicide was not a main factor to induce the herbicidal activity.

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